1. Field of the Invention
The present invention relates to a gas sensor having a protector for protecting a detection element exposed to exhaust gas from adhesion of water.
2. Description of the Related Art
Conventionally, a gas sensor is known having a detection element which generates an electromotive force of a magnitude corresponding to the concentration of a specific gas component; for example, NOx (nitrogen oxides) or oxygen, contained in exhaust gas of an automobile or the like or whose resistance varies with concentration of the gas component. The gas sensor is attached, for use, to an exhaust gas pipe of an automobile or the like. Because the detection element is exposed to a high temperature exhaust gas, subjecting the detection element to thermal shock caused by adhesion of water contained in the exhaust gas may crack or break the detection element. In order to address the above-noted problem, the gas sensor is fit with a protector for covering the detection element, thereby protecting the detection element from adhesion of water (refer to, for example, Patent Document 1).
The protector of the gas sensor of Patent Document 1 has a dual structure consisting of an outer protector and an inner protector. A front-end circumferential portion (a lower circumferential portion as viewed when the gas sensor is attached) of the outer protector has outer introduction holes for introducing exhaust gas toward a clearance between the outer protector and the inner protector (hereinafter, also referred to as the “gas separation chamber”). Guide portions are provided at the respective outer introduction holes of the outer protector so as to cause exhaust gas introduced into the gas separation chamber to swirl around the outer circumferential surface of the inner protector. Inertial force associated with the swirl separates a gas component from water which is heavy relative to the gas component. A rear-end circumferential portion (an upper circumferential portion as viewed when the gas sensor is attached) of the inner protector has inner introduction holes for introducing the gas component, which is separated in the gas separation chamber, into the interior of the inner protector.
The inner protector of the gas sensor of Patent Document 1 has a bottom portion projecting from the front end of the outer protector and has an exhaust hole formed in the bottom portion. Thus, a gas component of exhaust gas which is introduced into the inner protector can be exhausted from the exhaust hole.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2004-109125, which corresponds to US 2005/241937A1.
3. Problems to be Solved by the Invention
However, in the gas sensor of Patent Document 1, the exhaust hole of the inner protector has a size which ensures exhaust-gas exhaustibility. Thus, in some cases, a water droplet or the like can enter the inner protector through the exhaust hole. Since a front end portion of the detection element is directly visible from the outside, a water droplet which enters through the exhaust hole may adhere to the detection element. If the detection element is subjected to thermal shock caused by such a water droplet, the detection element may crack or break.